Electrically conductive adhesives are known in the prior art and are exemplified by the compositions disclosed in U.S. Pat. Nos. 4,652,398 and 4,557,860. In these prior art adhesives electrical conductivity is achieved by incorporating a conductive filler, typically silver powder or silver flakes, in the adhesive. These conductive fillers are generally present in an amount of at least 50% by weight of the total adhesive composition.
The use of such high loadings of conductive fillers is required to achieve the necessary degree of conductivity. However, the use of such high concentrations of conductive fillers can result in the loss of adhesive strength with increased temperature, difficulty in mixing and spreading the adhesive, short-working life of mixed components and tendency to sag during cure. In addition, there is tendency for such adhesives to exhibit a loss of conductivity after curing at elevated temperatures. This tendency of prior art adhesives to lose conductivity upon adhesive post-cure is attributed to surface oxidation of the filler.
There therefore exists a need for an electrically conductive adhesive which has high temperature property retention, good-working life, good mixing and application characteristics, and, in addition, is able to withstand high-shear forces and maintain its conductivity after elevated temperature curing.